Field of the Invention
Embodiments disclose a supercapacitor with a high-performance lithium-alloy anode, and in particular a supercapacitor with a boron doped lithium-alloy anode and a highly porous carbon (C) cathode.
Background of the Related Art
The expanding market of portable electronic devices and, especially, the emergence of electric vehicles and hybrid electric vehicles have created increasing demand for advanced energy storage techniques that can provide high energy and power densities and long cycling life. Two energy storage systems that are typically used in practical applications of portable electronic devices are lithium-ion batteries (LIBs) and supercapacitors (SCs). These two systems generally represent two extremes of the design space.
Generally, LIBs can deliver high energy densities (150-250 Wh/kg) by utilizing Faradaic reactions throughout the active materials comprising the batteries. However, this mechanism may lead to low power densities (<1000 W/kg) since solid-state ion diffusion in bulk electrodes is generally slow. LIBs may also suffer from short cycling lives (<1000 cycles) due to degradation of material structures.
On the opposite extreme, SCs typically offer high power densities (˜10,000 W/kg) because of the fast physical sorption rates of charges on the surfaces of active materials comprising the capacitors. This mechanism may also enable long cycling lives (>100,000 cycles) because it generally does not cause major structural changes. However, as only the surface is typically utilized, the energy densities of SCs are very limited (e.g., 5-10 Wh/kg).
Hybrid supercapacitors or supercapacitor-battery hybrid energy storage systems have been proposed as a way to incorporate the advantages of both LIBs and SCs into one system. Existing supercapacitor systems consist of SC electrodes (activated carbon) as cathodes to ensure high power density through adsorption/desorption of anions and LIB electrodes as anodes to provide high energy density by lithium (Li) insertion/extraction in a non-aqueous electrolyte. (See FIG. 1). For example, an energy density of 147 Wh/kg at 150 W/kg may be achieved by coupling a graphene-based three-dimensional porous carbon cathode and a Fe3O4/graphene nanocomposite anode. However, with such prior art systems, high energy densities are only achieved at very low power densities, and energy densities generally decrease significantly with increasing power densities.
An ideal anode in a hybrid supercapacitor system should have the following features: 1) the working voltage should be low so that the system is able to fully utilize the voltage window of the electrolyte to enable high energy density; 2) the anode should have high specific capacity to increase the energy density; 3) the anode should have excellent rate capability to match the high-power cathode to achieve high power density; and, 4) the cycling life should be long to improve cycling stability of the hybrid system.
None of the anode materials used in the prior art of hybrid supercapacitor systems meets all of these requirements. For example, Li4Ti5O12 and TiO2 have good cycling stability but high voltage (1.5 V) and low capacity (around 200 mAh/g). Graphite, on the other hand, shows the lowest lithiation voltage (0.1 V), but also low capacity (370 mAh/g) and mediocre rate performance.
Silicon nanostructures, such as nanowires, nanotubes, and nano/micro-sized particles have been used in hybrid supercapacitors in an attempt to achieve the benefits described above, but preparation of Si nanostructures may involve chemical/physical vapor deposition or highly toxic HF etching, which may incur additional costs.
Consequently, a supercapacitor system with high energy density at high power density, along with long cycling life, has not yet been demonstrated with the prior art. One reason for this is due to a lack of high performance anodes employed with the supercapacitor system.